Problem/Motivation

During installation a profile can override configuration provided by modules. However this is limited to install time. If a profile wants to provide a different default for a module that is installed after the profile installed it can't. This was a designed limitation of the ConfigInstaller. However, it is apparent that we have use cases for this. Not least with the testing profile in core which wants to disable translation download. To do this it has the file core/profiles/testing/config/optional/locale.settings.yml. This currently does not work, which results in the test run downloading translations unnecessarily.

Another upside of this change is that we become more friendly to distributions as they can override configuration for modules they don't enable but want to provide a specific opinionated default config for in case the user does install that module.

Discussed with alexpott and agreed that this makes sense as an RC target. There is no direct disruption for RC, just expanded functionality, but it would not be advisable to change the behavior after RC.